Control system

ABSTRACT

A moisture control system for use in paper making is disclosed as employing a traversing moisture sensor, the traversal time of which is made compatible with the thermal time constant of a dryer stage for such paper.

Bartles et a1.

Dec. 19, 1972 CONTROL SYSTEM Edward C. Bartles; Raymond A. Fraim, bothof Rochester, NY.

April 8, 1970 Inventors:

Filed:

App]. No.:

Related U.S. Application Data Continuation of Ser. No. 778,395, Nov. 25,1968,

Pat. No. 3,518,775.

U.S. Cl ..165/12, 165/26 [56] References Cited UNITED STATES PATENTS3,450,343 6/1969 Donath ..165/12 Primary ExaminerCharles SukaloAttorney-WalterO. Hodsdon and Robert F. Cody [57] ABSTRACT A moisturecontrol system for use in paper making is disclosed as employing atraversing moisture sensor, the traversal time of which is madecompatible with the thermal time constant of a dr er sta e for such 1m.Cl ..Fb 29/00 paper y g Field of Search ..165/12, 26, 27; 34/48 7 l 2Claims, 3 Drawing Figures .Sfeam H 54 M Moisture M2 Transin/ Timer v Io-- Reject vi] 20"0 20% P i Q 58 32 Mala! l 52 56 P s fl P2 Transient 77Reject w:

I I J "9 Q I F er l I j 44 /O"b I Bis/able Circa/l CONTROL SYSTEM Thisapplication is a Continuation In Part of U.S. Ser. No. 778,395 filedNov. 25, 1968 now US, Pat. No. 3,518,775, priority of which is claimed.

. BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to automatic control systems;

and in particular to automatic temperature control systems. As presentlyemployed, the invention is embodied in a system for the manufacture ofpaper, serving to regulate the moisture content of such paper. Theexpression Time Constant and especially as it applies to, thermal timeconstant within this specification, is defined as the time that it takesfor an element, at one temperature, to be changed to a secondtemperature assuming that the rate of change of temperature between suchfirst and second temperatures is maintained constant. For more onthermal time constants, see Physics Shortly and Williams, Prentice-Hall,Inc., N.Y., 1950, page 360-362.

2. Description Relative to the Prior Art The moisture content of paperduring its manufacture has a direct bearing on the quality of suchpaper. To keep the moisture level of paper within certain prescribedlimits, the paper is passed through a thermally controlled dryer stage,which quite commonly is steam operated. The particular system over whichthe present invention provides an improvement employs automatictemperature control of its dryer stage, doing so by sensing papermoisture and, in response thereto, controlling the flow of steam intothe dryer stage. The problem with such an arrangement is that themoisture level of paper is subject to frequent transient variations; andsince steam control of the dryer has an inherently long time constant,i.e., the initial thermal impact of steam is barely reflected in a dryertemperature change, the instantaneous dryer temperature is rarely at thelevel that it ought to be for a given moisture condition. (It should benoted that longtime constants, while especially troublesome in asteam-dependent system, are also apparent in other forms of dryingapparatus.) As a result, steam is continually applied to the dryer asthe automatic control system in question follows up and hunts for itsnull. I

It has been suggested that the particular problem of the prior artsystem under discussion may be obviated by the incorporation therein ofappropriate lead and/or lag devices or circuits whereby, say,anticipatory quantities of steam may be applied to the dryer in responseto moisture representative signals. While such an expedient wouldundoubtedly work, it would require appreciable and costly modificationsto the moisture control system.

Paper manufacturers often employ a traversing moisture sensor in theirsystems for moisture-recording purposes. U.S. Pat. Nos. 3,214,845(Huffman) and 2,951,007 (Lippke) indicate such systems; and similarly,the system modified by means of the invention also includes a traversingsensor, viz. a Lippke sensor.

SUMMARY OF THE INVENTION Moisture control according to the inventionprovides for a cyclic sampling of moisture in a moving paper web; andsuch sampling is at a rate which is compatible with the inherent timeconstant of the moisture control system. (The expression compatible withas used herein means "equal to or greater than.) Apparatus according tothe invention implements this concept by sensing for moisture at apreselected widthwise location (window) of the paper web; and bymatching the traversal time of its (Lippke) sensor to the system timeconstant. It will be appreciated that, in a properly operating system,moisture content and dryer temperature (relative to respectivereferences) vary as direct functions of each other: By cyclicallysampling moisture, a comparison is, in effect, cyclically made betweenan instantaneous dryer temperature and a commanded temperature, the timedelay inherent in changing the dryer temperature being effectivelycancelled by such cyclic comparisons. The invention contemplates usingsuch cyclic comparisons to control tightly the above-notedmoisture-temperature relationship on a cycle to cycle basis.

An object of the invention is to provide an automatic control system,the inherent time constant thereof being effectively cancelled bycyclically controlling such system at intervals proportional to suchtime constant.

Another object of the invention is to provide an automatic controlsystem which cyclically adjusts the temperature of an element thereof atintervals proportional to the time that it takes for a regulator thereinto influence such temperature in a commanded manner.

Another object of the invention is to provide an automatic system forcontrolling the moisture content of paper during its manufacture, suchsystem being adapted to regulate the heat applied to a dryer stage forsuch paper by cyclically sensing the moisture within such paper atintervals proportional to the time constant for applying heat to thedryer stage.

Another object of the invention is to use the traverse time of atraversing moisture sensor to cancel the effect of delays inherent inthe application of heat to a dryer within a paper-making system.

The invention will be described with reference to the figures, whereinFIG. 1 is a block diagram illustrating a control system according to theinvention,

FIG. 2 is a schematic illustration showing the invention incorporated ina paper-making system; and employed to control automatically themoisture content of such paper, and

FIG. 3 is a block diagram of a presently preferred form of paper-makingsystem, as in FIG. 2.

As above stated, the invention can find many and varied uses. FIG. 1 hasbeen provided for the purpose of describing some of the broadimplications of the invention; and to facilitate an understanding of thedetails which are indicated in connection with FIGS. 2 and 3.Corresponding parts found in FIGS. 1, 2, and 3 are, respectively,provided with no, single, and double primes.

With reference to FIG. 1, a controller 10 and responder 12 cooperate tocause the responder to track a reference condition established by asource 14 thereof. When the controller 10 commands the responder 12 tochange its condition, a time delay T. D. obtains before such responder12 can fully react to its input. To assure that the responder 12 isfully responl060l I 0496 sive to a given input command before respondingto any subsequent (e.g., transient) input(s), the invention providesthat the controller 10 apply subsequent input command(s) to theresponder 12 only if the previous command(s) thereto was (were)insufficient to bring the responder 12 to the reference condition. Thisrequires that the controller 10 be cyclically operated, at intervalscorresponding to the time delay T. D., whereby theeffect of such timedelay is eliminated from the system. An AND gate circuit 16 cyclicallyturned on by means of a switch 18 operates the controller 10 inaccordance with the error e (comparator 20) between the reference 14 andresponder 12 conditions (pick-off 22).

With the above as background, reference should now be had to FIG. 2which shows the concepts of FIG. 1 practiced in the manufacture ofpaper; and wherein the cyclic traversal of a moisture sensor is employedto advantage to provide cyclic gating of error signals:

A paper web 24 is conveyed, during its manufacture, over a dryer drum 12to which steamis applied in amounts depending on the moisture content ofthe paper. A sensor 22', driven by means of a (variable speed) motor 28,travels back and forth, widthwise across the paper web 24, registeringthe moisture content of such paper. A transducer 18, which may be asimple suitable tapped potentiometer, produces a gate signal wheneverthe moisture sensor 22 occupies a certain widthwise position of the web24, as indicated by the dashed window 26. A pair of threshold circuits20'a and 20'b receive the moisture-representative signal from the sensor22. If the moisture within the paper is above a certain amount, asdetermined by a highmoisture reference which is set into the thresholdcircuit 20'a, such circuit produces an output signal; if such moistureis below a certain amount, as determined by a low moisture referencewhich is set into the threshold circuit 20b, such circuit produces anoutput signal. Output signals from the threshold circuits 20a and 20'bare respectively applied to AND gate circuits l6'a and 16'b which areboth cyclically turned on whenever the moisture sensor 22' occupies awindow 26 position. In the event that the paper moisture level is toohigh when the sensor 22 is at a window 26, the gate circuit 16'a appliesa signal to operate a valve (27)-actuating temperature controller la forincreasing the steam into the dryer 12; if the moisture level is too lowwhen the sensor 22' occupies a window 26, the gate circuit l6'b appliesa signal to a valve (27 )-actuating temperature controller l0'b fordecreasing the steam into the dryer 12'.

As is known, the application of heat, say by means of steam into aplenum, has an inherent time constant; and such is symbolicallyrepresented on FIG. 2 as T. D.(s). Therefore, to equate paper moistureand dryer temperature, as noted above, the invention proposes that thespeed of the motor 28 be so set that the traversal time of the sensor22', indicated on FIG. 2 as T. D.(t), matches the time constant T.D.(s). In this way, each window sampling of moisture becomes, in effect,a comparison of the instantaneous dryer 12' temperature with a referencedryer temperature for a given moisture condition; and each windowsampling serves to set the temperature controllers 'a and l0b until thenext such sampling. Thus, the temperature controllers 10'a and 10b andvalve 27 do not continually, futilely, hunt in-between, and during,successive window sampling.

It should be noted that the temperature controllers l0a, l0'b may,without departing from the spirit of the invention, either control theapplication of heat in proportion to the magnitude of output signalsfrom the threshold circuit 20'a, 20'b, or simply by preset discreteamounts whenever the threshold circuits 20a, 20'b produces outputsignals. This latter technique has been incorporated in the presentlypreferred embodiment of the invention, and which embodiment is indicatedin FIG. 3:

Reference should now be had to FIG. 3. A traversing Lippke moisturesensor 22" is driven to and fro across the width of a paper web 24" bymeans of a motor 28". The Lippke sensor produces two signals: onerepresenting the moisture content of the paper web immediately beneaththe sensor 22"; the other representing the lateral position of thesensor 22" with respect to the web 24". A steam operated dryer 12" isemployed to control the moisture content of the paper web 24".

The position and moisture signals are applied to respective indicatorcontrol meters 40,42 such, for example, as those manufactured by APIInstruments Company, 7100 Wilson Mills Road, Chesterland, Ohio, Model603K. Meters such as these are provided with adjustment screws30,32,34,36 for setting high and low limit markers M M P P The screw 30may be set so that the meter 40 produces a line 20"b signal whenever thepointer of the meter 40 falls below a certain threshold moisture level Msimilarly the screw 32 may be set so that the meter 40 produces a line20"a signal whenever the pointer of the meter 40 goes above a certainthreshold moisture level M The widthwise window of the paper web 24"which is to be examined for moisture is likewise selected by setting thescrews 34,36. As the pointer of the meter 42 enters (P the selectedwindow, a signal is applied to a bistable circuit 44,'e.g., a flip-flopcircuit and complementing relay, which closes the relay switches 16"aand 16"b; as the pointer of the meter 42 leaves (P,) the window, asignal is applied to the bistable circuit 44 causing the relay to dropout, and its switches 16"a, 16"b to open. In a properly operatingsystem, the pointer of the meter 42 will oscillate back and forth,tracking the position of the Lippke sensor 22"; and the pointer of themeter 40 will reside within the boundary defined by the markers M M, ofthe meter 40.

It should be appreciated that the meter 42, bistable circuit 44 andswitches 16"a, 16"b cooperate to form an AND gate circuit 18" which iscyclically turned on at intervals defined by the speed of the motor 28";and such intervals are deliberately made compatible with the time thatit takes for steam to be conveyed to the dryer 12", as discussed above.

In its presently preferred form, the moisture representative signalsfrom the meter 40 are applied through respective transient rejectfilters 50,52, wherein short term spurious moisture signals areeliminated. While electronic low-pass filters would work for thispurpose, the preferred form of the invention employs electronic clocksas inexpensive filters. Each such clock is triggered into operation byan input from the meter 40, and produces an output signal only l060ll0497 if the duration of its input signal is greater than a certainamount, thereby eliminating short lived moisture signals. Clock circuitssuch as those manufactured by industrial Timer Corporation, 351 U. S.Highway 287, Parsippany, New Jersey, Model 608 are recommended.

It is indicated above that in this, the preferred form of the invention,steam is, for sake of simplicity, applied to the dryer 12" in presetdiscrete amounts. To this end, the invention employs'timing circuits54,56 for operating a motor 58 for a preset time, in either of twodirections to open or close, respectively, a valve 60. The durationsdefined by the timers 54,56 are so selected that in the event themoisture limits (M M are passed through, the change in steam to thedryer 12", commanded at a given meter 42 window, will be just aboutsufficient to position the pointer of the meter 40 approximately mid-waybetween its markers M M prior to the start of the next meter 42 window.And since the duration between the windows is made to match the delayedeffect of dryer steam, just one such application or exhausting of steamis all that is ever necessary to restore, without hunting, the desireddryer temperature and web moisture conditions.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention. A typical variation, for example, would be to disable thedrive for the traversing sensor 22", thereby to freeze the sensor at agiven widthwise location of the web 24". It would then become necessaryto simulate the sensor traversal time, for example, by cyclicallyoperating the switches l6"a and 16"b, say by means of a free-runningpulse generator.

What is claimed is:

1. Control apparatus comprising a. first means having a regulatablecondition,

b. signal responsive second means for regulating said condition andcooperative with said first means, and having therewith a certain timeconstant,

c. third means, cooperative with said first means, for producing asignal variable as a function of such condition,

d. fourth means, cooperative with said third means, for applying thesignal of said third means to said second means, and

e. fifth means for cyclically operating said fourth means at intervalsproportional to said time constant.

2. The apparatus of Claim 1 wherein said first means is a heatexchanger, wherein the regulatable condition of said first means is itstemperature, and wherein said second means is adapted to regulate theflow of thermal energy into said heat exchanger.

1. Control apparatus comprising a. first means having a regulatablecondition, b. signal responsive second means for regulating saidcondition and cooperative with said first means, and having therewith acertain time constant, c. third means, cooperative with said firstmeans, for producing a signal variable as a function of such condition,d. fourth means, cooperative with said third means, for applying thesignal of said third means to said second means, and e. fifth means forcyclically operating said fourth means at intervals proportional to saidtime constant.
 2. The apparatus of Claim 1 wherein said first means is aheat exchanger, wherein the regulatable condition of said first means isits temperature, and wherein said second means is adapted to regulatethe flow of thermal energy into said heat exchanger.